What was found
AIRCRAFT WAS NOT GROUNDED: AT TAKEOFF PLUS 3 HOURS AND 15 MINUTES HAD A MASTER WARNING FOR "SMOKE LAVATORY SMOKE" YET FLIGHT ATTENDANT REPORTED NO PASSENGER IN LAVATORIES EXCEPT FOR OLDER PASSENGER IN ONE AND HE WAS NOT SMOKING. ALL LAVATORIES CHECKED FOR RESIDUAL SMOKE NOTHING ANOMOUSLY NOTED. WARNING LASTED APPROXIMATELY 4 SECONDS. MAINTENANCE ACCOMPLISHED BITE CHECKS OF THE LAVATORY SMOKE DETECTION SYSTEM WITH NO FAULTS NOTED AND INSPECTED ALL OF THE LAVATORTIES AND FOUND NO SIGNS OF SMOKE IN ANY OF THEM.